A fused optical fibre coupler can be formed by taking two or more optical fibres, twisting them around each other a few times, and heating the twisted portion while pulling so that the fibres along that portion taper and fuse together. In a coupler of this type light signals propagating in say one of a number of optical fibres can be coupled into a number of other fibres. An example of the way in which fused optical fibre couplers can be formed is described in European Patent Application No. 174014 (Hitachi)
Fused optical fibre couplers are attractive for use in optical fibre communications networks because they have low loss, good temperature stability, mechanical rigidity and ease of manufacture. However the coupling ratio of these devices is wavelength dependent. For example in a coupler where say one port couples to two ports, the splitting ratio between the two receiver fibres may be 50/50% AT 1.3 .mu.m and anywhere between 80% to 20% and 99% to 1% at 1.52 .mu.m depending upon whether the fibres are lightly fused or well fused. There is clearly a need for an optical fibre coupler in which the splitting ratio is not critically wavelength dependent and it is an object of the present invention to provide such a coupler.
According to one aspect of the present invention there is provided an optical fibre coupler in which light propagating in one fibre is arranged to be coupled into one or more other fibres, wherein the propagation constant of the receptor fibre or fibres in the coupling region is arranged to differ slightly from that of the one fibre. The propagation constant of an optical fibre is a characteristic which can be defined, for optical radiation of a given wavelength propagation in the fibre, as the angular frequency divided by the velocity of a point of constant phase within the fibre.
It has been found that in a coupling between two fibres of slightly different propagation constant the maximum coupled power from one fibre to the other can be made to have a value less than 100%. By appropriately selecting the difference in propagation constant the maximum coupled power can be arranged to have for example a value of 50%. Furthermore at the maximum value the variation of coupled power with wavelength is at its least sensitive so that the coupler will have that maximum coupling value over a relatively wide operating wavelength range. Thus it is possible to construct an optical fibre coupler in which light propagating in one fibre is split into two fibres with the splitting ratio between the two receptor fibres being essentially constant over a relatively wide operating wavelength range.
The difference in propagation constant can be achieved by using fibres of different diameter or fibres of different profile or by tapering one of two identical fibres more than the other. The coupler may be a fused fiber coupler.
According to another aspect of the present invention there is provided a method of forming a fused optical fibre coupler in which two fibres are twisted together along a portion of their length, the propagation constants of the fibres in the twisted region being slightly different, optical radiation is launched into one or both of said fibres whilst the twisted region is heated and pulled, and the optical radiation transmitted through the coupling region is monitored.